The removal of sulfur compounds from crude oil and its fractions has been of significant importance for several decades, but has become even more important in recent years due to tightening environmental legislation. While much of the prior art focuses on the desulfurization of individual crude oil fractions, a large segment of the art today and in the past has addressed the requirement for hydroprocessing of whole crude oil. The majority of the interest in recent years has been on the upgrading of very heavy (API gravity <20) crude oil, shale and tar-sands to produce light sweet synthetic crudes. The major driving force for these processes is the demand for light crude oils in refineries and the low value of highly viscous feedstocks. Furthermore, demand is shifting from high sulfur fuel oils to low- and ultra-low sulfur products, 1 wt % (LSFO) and 0.5 wt % (ULSFO). Therefore, the ability to produce LSFO or ULSFO instead of high sulfur fuel oils is advantageous.
One of the major technical challenges posed when hydrotreating heavy oil fractions or whole crude is the effect of minor concentrations of contaminants such as organic nickel and vanadium compounds. These organometallic components have been proven to detrimentally impact the activity of hydrotreating catalysts.
Another major challenge faced by processing whole crude oil is that the concentration of coke precursors is very high. These coke precursors, such as asphaltenic plates detrimentally impact the activity of the hydro-desulfurization (HDS) catalysts in question. This means that the performance of a conventional process would decrease over time, requiring catalyst replacement to ensure continued profitable operation. This catalyst replacement can be costly and also time consuming, significantly impacting the economic feasibility of such a process.
Conventionally, hydro-demetallization (HDM) and HDS reactors are arranged and connected in series with the feedstock being processed sequentially by each of the reactors. The reactant species within the feedstock; therefore, have a concentration gradient when moving in an axial position down the catalyst bed, continuing in subsequent reactors. This concentration gradient causes the catalyst in the reactor where the feed is first incident to experience the highest concentration of reactant species.
Consequently, the lifetime of the catalysts in the reactor depends on their position in the loading scheme, with the catalysts at the inlet of the first sequential reactor experiencing a significantly higher concentration of deactivating metal compounds than the equivalent catalysts at the outlet of the final sequential reactor. Consequently, in conventional reactor systems, the catalysts at the inlet of the first HDM sequential reactor would be, at some point in the cycle, significantly deactivated whilst catalysts at the outlet of final sequential reactor would be deactivated to a significantly lower extent. Additionally, as catalysts become contaminated with these metals, the catalyst performance decreases and the possibility of pressure drop increases, which further increases operating costs. And because typical HDM and HDS systems are constricted in a sequential fashion, the unit must be shut down in order to replace or regenerate or rejuvenate the spent catalysts. These two phenomena can significantly reduce the on-stream cycle length, which negatively impacts the operating costs. In the event of one or both of the above phenomena, the on stream cycle is terminated prematurely whilst the catalyst in the second reactor has not be fully utilized.
Therefore, there is a need for a safe and cost effective solution to decreasing time spent replacing or regenerating spent catalyst, while at the same time fully utilizing all of the catalysts within a given reactor.